A method is provided for fabricating a semiconductor structure. The method includes providing a semiconductor substrate, and forming a shallow trench isolation structure in the semiconductor substrate. The method also includes forming a plurality of parallel gate structures on the semiconductor substrate surrounded by the shallow trench isolation structure. Further, the method includes forming a plurality of first trenches in the semiconductor substrate at least one side of the gate structures proximity to the shallow trench isolation structure, and forming a first silicon germanium layer with a first germanium concentration in each of the first trenches. Further the method also includes forming a plurality second trenches in semiconductor substrate at least one side of the gate structures farther from the shallow trench isolation structure, and forming a second silicon germanium layer with a second germanium concentration greater than the first germanium concentration in each of the second trenches.